For the high-Tc superconductors, there are several experimental and
theoretical reasons for seriously
questioning traditional BCS theory with a simple
phonon-induced electron-electron interaction. Experimentally, an
extremely small isotope effect measured for
is often cited as one such deterrent [17].
BCS theory predicts an isotope
shift if Tc is determined by the motion of the oxygen ions; however,
substitution of 18O for 16O in
does
not significantly change Tc.
Ruling out a phonon-induced pairing mechanism based on these observations
is premature however.
The presence of an isotope shift implies
that the lattice is certainly involved in the pairing mechanism.
However, one cannot assume that the lack of an isotope effect
necessarily
implies that the pairing mechanism does not involve
phonons. Superconductors such as ruthenium and zirconium exhibit virtually
no isotope effect, while uranium shows a negative isotope effect
[46]. The problem is that there are many additional factors
which can effect the strength of the isotope shift. Thus, by itself
the lack of a
significant isotope effect in
is not enough
to rule out an electron-phonon mechanism.

The anomalous normal-state properties of the cuprates suggest
that these materials are not just a normal Fermi liquid above Tc, and
therefore may not be adequately described by BCS theory below Tc.
The electrical dc resistivity ,
exhibits a linear dependence in temperature over a wide range of temperatures
above Tc. For a conventional Fermi liquid associated with normal
metals,
.
This is a manifestation of the long lifetime
of electrons near the Fermi surface in a conventional Fermi liquid
[47]. The nuclear spin-lattice relaxation rate
T1-1 (T)shows a temperature dependence substantially different from that of normal
metals. Other anomalous normal-state properties of the copper-oxide
superconductors include the thermal conductivity
,
the optical
conductivity
,
the Raman scattering intensity ,
the tunneling conductance as a function of voltage g(V), and the Hall
coefficient RH (T) [48]. All of these normal-state properties
are quite uncharacteristic of the Fermi liquid usually associated with
the normal state of conventional superconductors. In fact, it is possible
that the unusual normal-state properties of the high-Tc compounds
cannot be appropriately described by a Fermi liquid.

Some argue on theoretical grounds that BCS theory cannot explain the high
transition temperatures of the cuprates. For example,
it has been suggested that an electron-phonon
mechanism probably cannot account for transition temperatures in excess
of 40 to 50K [49]. The magnitude of the electron-phonon
interaction required to generate a Tc comparable to that of
,
would substantially weaken the lattice.
This structural instability would greatly reduce the density of electron
states at the Fermi surface and hence destroy superconductivity [18].
This argument is not accepted by all.
It has been suggested that the calculations leading to the
above conclusion are not valid,
so that an electron-phonon
mechanism may still be the basis for
superconductivity in the high-temperature
superconductors [50].